In blow molding processes, molten resins must form into stable parisons for a time long enough to permit a mold to enclose the parison. If these molten polymers do not possess sufficient melt strength or melt viscosity, the extruded tube will tend to elongate or draw under its own weight so that the resulting blow molded article will have non-uniform wall thicknesses, low surface gloss, and poorly defined sample shape.
Polymers such as polyesters, polyamides, and polyacetals when melted, generally form thin liquids having low melt viscosities. These low melt viscosity materials are unsuited or are only poorly suited for the manufacture of extruded shapes, tubes, deep-drawn articles, and large blow molded articles. In order to overcome this disadvantage and to convert these polymers to a form better suited for the above-mentioned manufacturing techniques, it is known to add compounds to the plastics which will increase their melt viscosities. The materials which are added to increase the melt viscosity of the plastics are generally cross linking agents, as described, for example, in U.S. Pat. No. 3,378,532. Such cross linking agents may be added during the condensation reaction by which the plastics are formed, and/or to the plastics after their formation (prior to, or during their melting). Examples of cross linking agents which may be added to the plastics after their formation and before or after their melting in order to increase the melt viscosity include compounds containing at least two epoxy or isocyanate groups in the molecule, organic phosphorous compounds, peroxides, bishaloalkylaryl compounds, and polyesters of carbonic acid.
These known cross linking agents which are added to increase the melt viscosity of the polymer are not completely satisfactory. They may, for instance, cause an excessively rapid and large increase in viscosity of form reaction products which have an adverse influence on the quality of the plastics. Furthermore, the results obtained with the use of these known cross linking agents are not always uniform or reproducible. For example, when polyesters of carbonic acid are used to increase the melt viscosity, the degree of viscosity increase is generally dependent not only upon the amount of additive used but also upon its molecular weight and on the state of the polycondensation reaction at which the addition takes place.
It has been observed that besides having sufficient melt viscosity or melt strength, polymers which are to be used in blow molding and related applications should also possess in certain instances sufficient die swell, i.e., the molten polymer should expand as it is released from the extrusion die. This die swell is important for blow molding applications where containers having a handle present thereon are to be formed.
It has been further observed that polymers having low relative viscosities are particularly difficult to blow mold and are also unsuited for many other related applications.
It has been still further observed that polymers which are utilized in blow molding applications should also possess a high degree of shear sensitivity, i.e., the molten polymer should thin out and become less viscous upon the application of increasingly higher rates of shear.
A shear sensitive polymer is more easily processed through an extruder and aids in providing an efficient blow molding process. Thus, a polymer having the combination of enhanced relative viscosity and shear sensitivity may be readily extruded (while being subject to a high degree of shear) and thereafter evidence a high degree of stability in the parison as a result of the high relative viscosity and reduced shear present therein.
Thus, although the prior art illustrates the use of numerous additives to modify various properties of certain polymers, the search has continued for improved processes for improving certain rheological properties of these polymers, particularly, the melt strength and shear sensitivity such that these polymers may be useful in blow molding and related applications.
For polyesters, this search has culminated in U.S. patent application Ser. No. 715,946, filed Aug. 19, 1976, now U.S. Pat. No. 4,071,503 by the same inventors of the present application, namely, N. Thomas, F. Berardinelli and R. Edelman which is herein incorporated by reference and which is a continuation-in-part application of U.S. patent application Ser. No. 558,982, filed Mar. 17, 1975, now abandoned.
In the former application, a process is provided for preparing a polycarbodiimide modified thermoplastic polyester of increased melt strength and intrinsic viscosity, which process comprises incorporating into a thermoplastic saturated polyester, having at least one carboxyl end group, at least one polycarbodiimide which polycarbodiimide both (a) is derived from at least one aromatic diisocyanate which is either unsubstituted or contains up to one methyl substituent on each aromatic ring, and (b) contains at least three carbodiimide units per polycarbodiimide molecule, by reacting the carboxyl end group of the polyester while in the molten state with the carbodiimide groups of the polycarbodiimide.
Thus, the present invention seeks to modify polyamide containing compositions to obtain properties comparative to those obtained by a similar modification of polyesters with polycarbodiimides.
Also, U.S. Pat. No. 2,284,896 discloses a process for reacting a broad class of compounds, some of which may include carbodiimide groups with an organic substance having a plurality of groups containing a reactive hydrogen. This patent does not, however, specifically disclose the reaction between a polyamide and a polycarbodiimide of the type described herein.